The present invention relates generally to filters, and more particularly, to industrial filters of the porous metal type. The filters of the present invention are particularly adapted for use as inertial filters and also, when provided with a semi-permeable membrane on the upstream surface, for use in the separation of liquids by reverse osmosis.
Porous metal filters are conventionally made by forming metal particles or powder into the desired filter shape using a volatile binder and/or pressure to render it form sustaining and then the metal particles are converted into a porous aggregate by heating or sintering at a temperature approaching the melting point of the particular metal, preferably in a controlled atmosphere. Conventional porous metal filters made by conventional techniques are frequently referred to as isopermeable, in that they have a generally uniform pore structure throughout. Such filters are particularly useful for filtering foreign particles from a fluid, the particulate material being entrained in depth in the filter pores as the fluid travels therethrough. When the pores become clogged, the filter is no longer usable and has to be subjected to a cleaning operation in order to render it again effective. Modification of such filters which will increase the period of operation of the filter before clogging is disclosed and claimed in my prior U.S. Pat. No. 3,728,061 and comprises forming the filter in layers of approximately equal thickness with the upstream layer having a larger pore size and providing a lesser degree of filtration and the downstream layer having a smaller pore size and a higher degree of filtration.
Porous metal filters made in accordance with the prior art have been used as inertial filters, sometimes referred to as concentrators, but have not been entirely satisfactory for this purpose. In the use of inertial filters, the fluid to be filtered is pumped parallel to the upstream surface of the filter and as it passes along the surface, a differential pressure across the wall causes a portion of the fluid to flow therethrough as clear filtrate. The problem with porous metal filters made in accordance with the prior art when used as inertial filters is that when formed to provide the required high degree of filtration and necessary mechanical strength, the filter creates excessive back pressure interfering with the removal of filtrate and more particularly, the filter is resistant to cleaning by blowback pulses which is necessary to keep the filter in continuous operation.
Accordingly, it is an object of my invention to provide an inertial filter providing a high degree of filtration and required mechanical strength while at the same time having sufficient permeability so that it will have an acceptable low back pressure, and being so constructed that blowback cleaning of the filter is rendered more effective.
This and related objects, which will be in part obvious and in part pointed out in more detail hereinafter, are accomplished in accordance with the present invention by forming the filter wall with a matrix of sintered metal particles of relatively large size and having relatively large interconnecting pores and a very thin subsurface portion adjacent the upstream surface with sintered metal particles of relatively small size producing interconnecting small pores just below the upstream surface.